Bile secretion is a major function of the liver which is frequently impaired in diseases of the liver resulting in the syndrome of cholestasis. The long term objectives of this grant, funded continuously by NIDDK since 1973, have been to characterize the basic transport mechanisms in hepatocytes at the cellular and molecular level that determine the secretion of bile and to define alterations in these mechanisms that result in cholestatic liver disease . In this request for extension of this MERIT award, the specific aims continue much as before. They are Aim 1-A: Mechanisms of Transcriptional Regulation of MRP4/Mrp4 and its role in the adaptive response to cholestasis Aim 1-B: To evaluate the role of basolateral Osta deletion/inhibition in kidney and intestine as a protective effect in cholestasis and hyperlipidemia. Aim 1-C: To evaluate the role of MDR3/Mdr2 as therapeutic targets forfenofibrate (FF) and all-trans retinoic acid in human and rat hepatocytes and, if so, by what mechanism. These aims are a continuation of our work to understand the molecular mechanisms for adaptive regulation of hepatocyte membrane that are important determinants of the adaptive response in cholestatic liver injury - and to devise new therapies based on this information. In particular in Aim 1-A and B we will continue to characterize the transcriptional regulators of the human MRP4 promoter as well as to continue to assess the role that the heteromeric, facilitated bile salt transporter, OSTo-OSTp plays in the adaptive response to cholestasis. Aim #1-C determines the molecular mechanisms by which fenofibrate improves cholestatic liver disease and the role of all-trans retinoic acid in this process. AIM # 2: (previously Aim #3) continues to characterize post-transcriptional mechanisms of regulation of the expression of canalicular ABC transporters by investigating the functional roles of interacting proteins involved in maintaining canalicular apical membrane structural polarity of Mrp2. AIM #3: Comparative studies of bile acid transport in marine vertebrates. We will continue to utilize marine animals as comparative models for bile acid transporters in the enterohepatic circulation.